Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track. A spindle motor rotates the disk at a high speed so that an air bearing forms between the head and the disk such that the head flies just above the disk surface.
When a power failure occurs, it is desirable to park the head before the air bearing dissipates, such as by unloading the head onto the ramp near the outer diameter of the disk. It may also be desirable to finish a current write operation and/or to flush a write cache to the disk prior to parking the head. When the supply voltage is lost due to a power failure, the momentum of the disk spinning generates a back electromotive force (BEMF) voltage across the windings of the spindle motor. Disk drives will typically boost this BEMF voltage using a suitable voltage booster, and utilize the boosted voltage to power the control circuitry used, for example, to finish a current write operation by flushing cached write data to a non-volatile semiconductor memory. The BEMF voltage is also typically used as a current source for the switches of a H-bridge driver that drives the VCM for parking the head (e.g., by unloading the head onto a ramp).